RESUMO
The atomic and electronic structure of nonstoichiometric amorphous tantalum oxide (TaO x ) films of different composition has been investigated by means of electron microscopy, x-ray photoelectron spectroscopy, Raman and infrared spectroscopy. The dispersion of the absorption coefficient and refraction index has been studied by spectral ellipsometry. The optical spectra were interpreted using the results of a quantum-chemical simulation for crystalline orthorhombic TaO x . It was found that the presence of oxygen vacancies in the oxygen-deficient TaO x film show an optical absorption peak at 4.6 eV. It has been established that TaO x consists of stoichiometric Ta2O5, metallic Ta clusters less than 20 nm in size, and tantalum suboxides TaO y (y < 2.5). The model of nanoscale potential fluctuations of TaO x bandgap in the range of 0-4.2 eV is proposed and justified. The design of the flash memory element based on the effect of localization of electrons and holes in Ta metallic nanoclusters in the TaO x matrix is proposed.
RESUMO
Optical and transport properties of nonstoichiometric tantalum oxide thin films grown by ion beam deposition were investigated in order to understand the dominant charge transport mechanisms and reveal the nature of traps. The TaOx films composition was analyzed by X-ray photoelectron spectroscopy and by quantum-chemistry simulation. From the optical absorption and photoluminescence measurements and density functional theory simulations, it was concluded that the 2.75 eV blue luminescence excited in a TaOx by 4.45 eV photons, originates from oxygen vacancies. These vacancies are also responsible for TaOx conductivity. The thermal trap energy of 0.85 eV determined from the transport experiments coincides with the half of the Stokes shift of the blue luminescence band. It is argued that the dominant charge transport mechanism in TaOx films is phonon-assisted tunneling between the traps.
